If you haven't seen the movie "Doing Time, Doing Vipassana," it's incredibly inspiring. And it's now on youtube in five parts. [youtube]bOoFXOWGC5o[/youtube]
For more on yoga and meditation in the prisons, this Wikipedia link has a lot of information. If you're interested in doing prison work here in the DC area, let me know and I can connect you with some very active and inspiring people.